Electrochemical fuel cells convert reactants, namely, fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst is employed to induce the desired electrochemical reactions at the electrodes. In addition to electrocatalyst, the electrodes may also comprise an electrically conductive porous substrate upon which the electrocatalyst is deposited. A particularly preferred fuel cell is the solid polymer electrolyte fuel cell, which employs a membrane electrode assembly ("MEA"). The MEA comprises a solid polymer electrolyte or ion-exchange membrane disposed between two porous electrode layers.
A broad range of reactants can be used in electrochemical fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be substantially pure oxygen or a dilute oxygen stream such as air. Both the fuel and oxidant streams are typically delivered to their appropriate electrodes via flow field plates. A flow field plate is located next to each electrode substrate on the side opposite the electrolyte/electrocatalyst interface. Fluid passages may be formed in the flow field plates (e.g., a pattern of fluid channels) such that fluid is essentially delivered over the entire porous electrode substrate. Flow field plates also typically serve as current collectors and as structural supports for the electrodes.
The output voltage of a single fuel cell under load is typically less than one volt. Therefore, in order to provide greater output voltage, usually numerous cells are stacked together and are connected in series to create a higher voltage fuel cell stack. End plates are placed at each end of the stack to hold the stack together and to compress the stack components together. Compressive force is needed for effecting seals and making electrical contact between various stack components. For various reasons, some resilience is needed in the compression end plate assemblies, for instance to accommodate and compensate for dimensional changes and to maintain compressive force over prolonged periods of time. Examples of various resilient compression end plate assemblies are disclosed in U.S. Pat. Nos. 5,484,666, 5,789,091, and PCT/International Publication No. WO95/28010 (Application No. PCT/CA95/00182).
Since the electrochemical reactions in a fuel cell are exothermic, often a cooling fluid is circulated at various locations throughout a fuel cell stack. (A liquid coolant, e.g. water, is commonly used, especially in high power fuel cell stacks.) Typically other additional flow field plates are employed to circulate the cooling fluid in thermal contact with components in a fuel cell stack. Thus, fuel cell stacks frequently comprise multiple flow field plates for three different fluids (i.e., fuel, oxidant, and coolant) and desirably comprise fluid manifolds to distribute these fluids to each flow field plate.
Preferably, each stack fluid is derived from a common supply and, where appropriate, each is ultimately discharged to a common exhaust. (Although substantially pure reactant streams can be dead-ended in a stack, even these are commonly exhausted periodically in order to purge accumulations of impurities.) One approach to connecting each flow field plate to a common fluid supply and exhaust is to internally manifold the fuel cell stack. That is, an external fluid is supplied to and exhausted from each respective flow field in the stack via common supply and exhaust ports. The ports in turn are connected to a pair of manifolds formed within the stack itself.
The two manifolds in turn are connected to opposite ends of each flow field in the stack. Preferably, the internal manifolds are designed such that the reactant pressure and flow are similar through each flow field in the stack.
In certain fuel cell systems, multiple fuel cell stacks are connected in a series and/or parallel array. Preferably the fuel cell stack manifolds are interconnected to form common array manifolds so that each of the fuel, oxidant, and coolant streams can ultimately be provided and discharged by a single connection to the entire array. Preferably, the "stack" manifolds are designed such that the fluid pressure and flow are similar through each stack in the array.
A conventional manifold arrangement for an array of fuel cell stacks employs a network of pipes, with larger pipes providing a common fluid supply (or exhaust) that connect in turn to smaller branch pipes, each at relatively similar pressures, leading to each individual stack. To obtain the most efficient flow of fluid, conventional piping with a circular cross section is preferred. Also, to withstand pressure using a minimum wall thickness, conventional round piping is preferred.
However, the volume occupied by such array manifolds can be substantial, often being much greater than the actual volume of the piping itself. It can be difficult to accommodate complex conventional piping networks in the space available in certain fuel cell applications.
For parts reduction and volume efficiency, stack manifolds can be incorporated into the compression end plates of the fuel cell stacks in the array. For example, U.S. Patent No. 5,486,430 shows an array manifold integrated into the compression end plates of multiple fuel cell stacks. Therein, the fuel cell system fluids flow through main array passages formed in the end plates which, in turn, are connected by branch passages to the inlets and outlets of internal manifolds of the fuel cell stacks. If the cross-sectional area of the main array passages is significantly larger than that of stack branch passages, a minimal pressure loss occurs along the length of the main array passages and thus the fluid flow to each stack branch is similar. However, the minimum thickness of an end plate comprising a circular main passage is of course limited to the diameter of the circular passage. Further, the minimum width of the thinnest end plate comprising a row of circular main passages is limited to the sum of the diameters of the circular passages in the row. To improve the viability of fuel cells as a commercial power source, it is generally desirable to improve the power density of the stack, that is, to reduce the stack dimensions and weight for a given electrical power output capability. Thus, where possible, it may be desirable to employ thinner and/or narrower manifold and end plate assemblies.
Although it adds complexity to a fuel cell system, it can be advantageous to periodically reverse the flow of fluids through the flow fields in a fuel cell stack (e.g., for purposes of distributing water to a solid polymer ion-exchange membrane in a fuel cell, as disclosed in U.S. Patent Application Serial No. 08/980,496, filed Dec. 1, 1997 by the same applicant as that for the present application). If possible then, it may also be desirable to incorporate flow reversing or flow switching mechanisms or devices into improved manifold assemblies.